1. Field of the Invention
The present invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply with primary side sensing of the power supply output voltage.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies or adapters are widely used to power electronic products as well as charge batteries used to power mobile products such as for example wireless phones, palm top computers, toys, etc. The output voltage of the power supply must be regulated to within a specified range depending on the product being powered. Typically this requires that the switched mode power supply includes components at the output of the power supply that sense the output voltage and provide feedback for a switched mode power supply controller or regulator which adjusts the power supply operation accordingly to maintain output regulation.
Known power supply techniques eliminate these output sense components and derive information regarding the power supply output voltage from the primary side of the power supply. However, the output voltage regulation possible with these techniques is compromised and is not acceptable to some types of products to be powered by the power supply.
A voltage sense circuit is disclosed. In one aspect, the voltage sense circuit may be employed in a power supply and includes a transformer including a sense winding and an output winding. A first diode is coupled to the sense winding, a first resistor is coupled to the first diode and a first capacitor coupled to the first resistor and the first diode. A second diode coupled to the first capacitor, the first resistor and the first diode. A second capacitor coupled to the second diode such that a voltage across the second capacitor is representative of a voltage across the output winding. In one embodiment, the first capacitor is discharged in a substantially shorter period of time than the second capacitor. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below.